Further characterization of the target antigens of two transmission blocking monoclonal antibodies (Mabs) in Plasmodium falciparum indicate that of the 3 proteins immunoprecipitated by these Mabs (255, 59 and 53 kilodaltons (kDa)), only the 255 kDa protein carries the epitopes with which the Mabs react, the 59 and 53 kDa proteins being apparently co-precipitated. Immunoradiometric assays confirm that the two Mabs, IIC5-B10 and IA3-B8, react with distinct epitopes on the target protein; the data is consistent with each epitope being represented once only on the target molecule. Following protease treatment and analysis of peptide fragments of the 255 kDa protein, no evidence was obtained of structural differences in this protein between several isolates of P. falciparum, including one isolate previously shown to be variant at one epitope on this protein. The two main surface proteins, 26 and 28 kDa, synthesized and expressed on zygotes of Plasmodium gallinaceum during transformation to ookinetes, of which the 26 kDa is the target of a transmission blocking Mab, were previously shown to be both glycosylated and to contain covalently bound fatty acid. However, based on differential precipitation by different Mabs and polyclonal immune sera, and different patterns of protease digestion fragments, the two proteins do not otherwise appear to be structurally related. A genomic expression library from P. falciparum is being screened for clones of E. coli containing recombinant DNA (in the lambda gt 11 vector) coding for gamete surface antigens using polyclonal and monoclonal antibodies against the sexual stages of P. falciparum. Transmission blocking antibodies have been successfully raised against a second species of human malaria, Plasmodium vivax, by immunization of rabbits with gametes of this parasite derived from natural human infections.